We have shown that lymphocyte migration is controlled by recognition of endothelial cells at sites of exit from the blood. Separate endothelial determinants expressed by high endothelial venules (HEV) in mouse lymph nodes and Peyer's patches, along with complementary lymphocyte "receptors", direct lymphocyte migration through mucosal versus non-mucosal tissues. Our long-term objective is to define the cellular and molecular mechanisms of lymphocyte-endothelial cell recognition, and to understand their regulation and their role in immune responses. Employing immunofluorescence, RIA, and immunohistologic screening procedures in combination with functional inhibition assays, we will attempt to generate rat monoclonal antibodies defining a) the lymphocyte surface "receptors" for lymph node or Peyer's patch HEV (we already have one monoclonal antibody, MEL-14, that appears to define the receptor for lymph node HEV as a unique 80,000 MW surface protein); and b) the endothelial determinants or molecules recognized by migrating lymphocytes. These molecules will be characterized biochemically by immunoprecipitation and affinity isolation; and functionally by their mimicry of or effect on lymphocyte-HEV interaction in vitro. We will determine the HEV-binding and in vivo homing specificities, and the level of HEV receptor expression, of defined lymphocyte subsets representing various stages of antigen-independent or antigen-dependent lymphocyte development; and we will ask whether non-lymphoid migratory cells (mast cells, monocytes, etc.) use the same or similar endothelial recognition systems. These studies will employ our standard assay of lymphocyte binding in vitro to HEV in lymph node or Peyer's patch frozen sections, in combination with short-term in vivo homing studies and quantitative dual fluorescence FACS analyses of lymphocyte staining with MEL-14. Demonstration of organ-specific lymphocyte-endothelial cell recognition has given the first insights into mechanisms controlling the differential migration of functionally important precursor and effector lymphocyte subsets. These mechanisms may determine the unique features of normal and abnormal (autoimmune, allergic) immune responses in mucosal versus non-mucosal tissues (such as the predominance of IgA plasma cells in the gut). Lymphocyte-HEV interaction is an outstanding model for the study of cell-to-cell recognition mechanisms, a fact that is illustrated by the preliminary identification of one of the molecules involved.